I have this code running in an always-on AutoHotkey script, listening for Ctrl + Up/Down/Left/Right combinations. When IrfanView has an open image, pressing the corresponding combination (Ctrl + one of the direction keys) will paste the image in your clipboard to that respective side. It's just a shortcut for Edit > Paste Special (add on side).
Code:
#IfWinActive ahk_class IrfanView ^Left:: KeyWait LCtrl Send {Alt}{E}{T}{L} Return ^Right:: KeyWait Ctrl Send {Alt}{E}{T}{R} Return ^Up:: KeyWait Ctrl Send {Alt}{E}{T}{T} Return ^Down:: KeyWait Ctrl Send {Alt}{E}{T}{B} Return